Final answer:
The contractile element in a twitch is the sarcomere, which is the functional unit of skeletal muscle. The sarcomere is composed of actin and myosin filaments that slide past each other during muscle contraction.
Step-by-step explanation:
In a twitch, the contractile element is the sarcomere. A sarcomere is the functional unit of skeletal muscle, and it is composed of two main protein filaments: actin and myosin. The actin and myosin filaments slide past each other during muscle contraction, resulting in the shortening of the sarcomere.